1. Field of the Invention
The present invention relates to an improved method of making newspaper printing plates. More particularly, the present invention relates to a method of making newspaper printing plates which eliminates failure of exact image superimposition in color printing, through strict control of positional relation between the image formed on each plate in a set of four color printing plates and bent edge stoppers formed along two edges of the plate.
2. Description of the Prior Art
FIG. 8 shows an example of a prior art rotary press for newspaper printing, which works in this manner. Streams of paper 1 unwinding from paper rolls in paper feeders A, A, A are printed on both sides in monochromatic printer sections B, B, B and a color printer section Ba. Three streams of paper 1, put one on another, are then sent in to a folder section C where they are cut and folded into one sheet of newspaper one after another and sent out through a delivery fan D or the like.
The color printer section Ba includes a common or center printing cylinder 2 and four blanket cylinders 3 and four color plate cylinders 4. On the four color plate cylinders 4 are mounted printing plates 5 which have an identical image thereon and are for printing in four different colors, respectively--yellow, red, blue, and black (for glossing). Such prior art arrangements are disclosed, for example, in unexamined Japanese patent applications laid open under Nos. H08-267724 and H08-290543.
Most commonly, the printing plate 5 is mounted on the color plate cylinder 4 in the following manner. The forward and backward edges of the printing plate 5 are bent to form stoppers 5a, 5b, which are put in slits 4a, 4a provided on the plate cylinder 4 for holding the printing plate as shown in FIG. 9. The printing plate is closely wound around the plate cylinder 4 and held that way.
The printing plate 5 is wound around the plate cylinder 4 and held in position. That is usually effected in this manner. An axial positioning hole 5c is provided in one or both of the bent edge stoppers 5a, 5b at specific points. Into the hole 5c is inserted a positioning protrusion 4b provided on the plate cylinder 4 so that the axial-directional position of the printing plate 5 is held at a predetermined place.
FIGS. 10, 11, and 12 show a typical prior art apparatus for forming bent edge stoppers 5a, 5b on printing plate 5. The apparatus has a printing plate loading table 7 with an edge blade 6, a plate presser 8 vertically movable in relation to the printing plate loading table 7, a movable bending tool unit 9 including bending tools 9a, 9b and a vertically movable positioning pin unit 10 on each side--the front and rear sides of the printing plate loading table 7.
After the printing plate 5 is placed on the printing plate loading table 7, positioning pins 10a are inserted into positioning holes 5d provided at the forward and backward edges of the printing plate 5--the positioning holes 5d place the printing plate 5 in position on the printing plate loading table 7. The plate presser 8 is moved down to clamp the printing plate 5 as shown in FIG. 11. Then, the positioning pins 10a are moved back downward and the bending tool 9a of movable bending tool unit 9 is turned by a drive unit (not shown). In that way, the forward and backward edges of the printing plate 5 are bent along the edge line of the edge blades 6, 6 to form bent edge stoppers 5a, 5b as shown in FIG. 12. This technique is disclosed in Japanese utility model application No. S53-159703.
The prior art bent edge stopper forming apparatus shown in FIG. 10 can form bent edge stoppers in the printing plate 5 for monochromatic printing with great efficiency and has proved to be highly practical. But the color printing plates processed by the prior art apparatus have often failed in exact image superimposition, presenting a number of problems in practical application.
To prevent the failure in exact image superimposition in color printing, it is necessary to position a set of four printing plates--for yellow, red, blue, and black--on the four color plate cylinders so accurately that the four images overlap almost perfectly. That is, care has to be taken in forming the positioning holes 5d in the four printing plates 5 that place the printing plate 5 in position on the printing plate loading table 7 so that the positional relation between the image T on each printing plate and the positioning holes 5d may always be maintained uniformly.
That is also applicable to formation of bent edge stoppers 5a, 5b and the axial positioning hole 5c in the printing plate 5. The positional relation between the image T on all four printing plates and the bent edge stoppers 5a, 5b and the axial positioning hole 5c must be maintained to be the same.
In the prior art of making a printing plate for newspaper printing, the positions of the positioning holes 5d for positioning the plate 5 on the table 7 are decided upon on the basis of the dimensions of the printing plate, center of the printing plate, or the like. Positioning holes 5d are formed at the positions thus decided upon. In other words, the prior art method of making a printing plate for newspaper printing neither checks nor controls the positional relation between the image T formed on the printing face of the printing plate 5 and the positioning holes 5d.
Some adjustment is made in forming the image T on the printing face of the printing plate 5 so that the images T on the four printing plates may be placed at the same position. But it is visual adjustment that is used, which is inexact. No precision positioning is involved.
The result is that the four printing plates 5 that form a set of color printing plates tend to be slightly different from each other in regard to positional relation between the image T on the printing face and the positioning holes 5d to position the printing plate 5 on the printing plate loading table 7, bent edge stoppers 5a, 5b, and axial positioning hole 5c. Then, it is impossible to prevent failure of exact superimposition of the color printing faces no matter how precisely the four color plates were adjusted in mutual positional relation on a color plate cylinder in a color printer section B.